(a) Technical Field
The present invention relates to a power train for a hybrid electric vehicle, more particularly, to a power train that includes two motors and an engine, and further includes a structure capable of reducing a motor capacity.
(b) Description of the Related Art
A hybrid electric vehicle, which is a type of eco-friendly vehicle, includes various power transmission devices using an engine and a motor as power sources. The power transmission devices have been developed and applied in various forms such as a series-type, a parallel-type, and a power split-type combining advantages of the series-type and the parallel-type.
The purpose of such hybrid electric vehicles is to provide a vehicle with high fuel efficiency and low cost. Accordingly, research has been conducted on a power transmission system capable of avoiding power source efficiency reduction, power transmission structure loss, and energy conversion loss, which are factors that may reduce fuel efficiency, and simultaneously achieving operation at a high-efficiency operating point.
An example of a conventional hybrid power transmission system is shown in FIG. 1 (RELATED ART).
As shown in FIG. 1, the power transmission system includes an engine ENG, two motors MG1 and MG2, and an over drive clutch and a power transmission gear therebetween.
In addition to EV mode in which the second motor MG2 of the two motors is directly connected to an output shaft, a generation mode in which a driving force of an engine is delivered to the first motor MG1 for generation through a simple gear, and an engine mode in which engine power is delivered to the output shaft through an Over Drive Clutch (ODC) engagement for high-efficiency point operation of the engine during high-speed driving may be implemented.
However, during HEV mode driving in which both engine and motor power are together used, since all driving force of the engine is outputted to the second motor MG2 after the generation of the generator MG1, energy conversion loss occurs twice, incurring reduction of the fuel efficiency. Particularly, since two high-capacity motors are used, the manufacturing cost may be significantly increased.
Another example of a conventional hybrid power transmission system is shown in FIG. 2 (RELATED ART).
As shown in FIG. 2, the power transmission system includes an engine ENG, two motors MG1 and MG2, and two planetary gear sets therebetween.
In addition to EV mode in which the second motor MG2 of the two motors is directly connected to an output shaft through the planetary gear, HEV mode in which an driving force of an engine branches through the planetary gear, allowing a portion of the driving force to branch to the output shaft and allowing the other portion of the driving force to branch to the first motor MG1, may be implemented.
However, in EV mode, since power is outputted through one planetary gear and two pairs of external gears, it is disadvantageous in terms of transmission efficiency. Also, in HEV mode, a portion of engine power branches through the planetary gear and thus there is little energy conversion loss. However, since two high-capacity motors are used, there is a limitation in that the manufacturing cost may be significantly increased.
Thus, there is a need for a new type of power transmission system which can improve fuel efficiency by reducing the energy conversion loss and can reduce the manufacturing cost by reducing the number of parts used in the power transmission system.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.